1. Field of the Disclosure
The present disclosure is in the field of pulsed neutron gamma ray excitation testing of geological formations. In particular, the disclosure determines the viscosity of a formation fluid from recorded spectra.
2. Description of the Related Art
Enormous amounts of hydrocarbon reserves occur in the form of heavy oil and tar sand bitumens produced by a process of biodegradation. In such deposits, significant vertical and lateral gradients in the oil composition and oil viscosity can result. In deposits of such sands, recovery of hydrocarbons may be done by steam injection in situ or by strip mining the deposits and processing the recovered material. An important aspect of the recovery operation is the amount of heat or steam needed, and the subsequent processing that is necessary. This, in turn, is directly correlated with the viscosity of the sands. In situ measurements of viscosity measurements are thus of great value in evaluating prospective deposits of oil sands and in the recovery of hydrocarbons from them. Gabitto and Barrufet have done an extensive study of the viscosity of heavy oils and developed a correlation model relating the viscosity to boiling point, API gravity and molecular weight. All of these determinations require recovery of an actual sample of the oil (a laborious procedure in itself) and further laboratory measurements.
NMR techniques have also been used for distinguishing between heavy oil and water. See, for example U.S. Pat. No. 6,755,246 to Chen et al., having the same assignee as the present disclosure. As noted therein, elevated temperatures are necessary to accomplish this. NMR methods are complicated, and the elevated temperatures necessary for the method of Chen adds further complication. The present disclosure addresses this problem using the novel approach of elemental analysis of gamma ray spectra made with a logging tool.
Well logging systems have been utilized in hydrocarbon exploration for many years. Such systems provide data for use by geologists and petroleum engineers in making many determinations pertinent to hydrocarbon exploration and production. In particular, these systems provide data for subsurface structural mapping, defining the lithology of subsurface formations, identifying hydrocarbon-productive zones, and interpreting reservoir characteristics and contents. Many types of well logging systems exist which measure different formation parameters such as conductivity, travel time of acoustic waves within the formation and the like.
One class of systems seeks to measure incidence of nuclear particles on the well logging tool from the formation for purposes well known in the art. These systems take various forms, including those measuring natural gamma rays from the formation. Still other systems measure gamma rays in the formation caused by bursts of neutrons into the formation by a neutron source carried by the tool and pulsed at a preselected interval.
In these nuclear well logging systems, reliance is made upon the physical phenomenon that the energies of gamma rays given off by nuclei resulting from natural radioactive decay or induced nuclear radiation are indicative of the presence of certain elements within the formation. In other words, formation elements will react in predictable ways, for example, when high-energy neutrons on the order of 14.2 MeV collide with the nuclei of the formation elements. Different elements in the formation may thus be identified from characteristic gamma ray energy levels released as a result of this neutron bombardment. Thus, the number of gamma rays at each energy level will be functionally related to the quantity of each element present in the formation, such as the element carbon, which is present in hydrocarbons. The presence of gamma rays at a 2.2 MeV energy level may for example, indicate the presence of hydrogen, whereas predominance of gamma rays having energy levels of 4.43 and 6.13 MeV, for example, may indicate the presence of carbon and oxygen respectively.
In these nuclear well logging systems, it is frequently useful to obtain data regarding the time spectral distributions of the occurrence of the gamma rays. Such data can yield extremely valuable information about the formation, such as identification of lithologies that are potentially-hydrocarbon producing. Moreover, these desired spectral data may not only be limited to that of natural gamma rays, for example, but also may be desired for the gamma ray spectra caused by bombardment of the formation with the aforementioned pulsed neutron sources.
Well logging systems for measuring neutron absorption in a formation use a pulsed neutron source providing bursts of very fast, high-energy neutrons. Pulsing the neutron source permits the measurement of the macroscopic thermal neutron absorption capture cross-section Σ of a formation. The capture cross-section of a reservoir rock is indicative of the porosity, formation water salinity, and the quantity and type of hydrocarbons contained in the pore spaces.
The measurement of neutron population decay rate is made cyclically. The neutron source is pulsed for 20-40 microseconds to create a neutron population. Neutrons leaving the pulsed source interact with the surrounding environment and are slowed down. In a well logging environment, collisions between the neutrons and the surrounding fluid and formation atoms act to slow these neutrons. Such collisions may impart sufficient energy to these atoms to leave them in an excited state, from which after a short time gamma rays are emitted as the atom returns to a stable state. Such emitted gamma rays are labeled inelastic gamma rays. As the neutrons are slowed to the thermal state, they may be captured by atoms in the surrounding matter. Atoms capturing such neutrons are also caused to be in an excited state, and after a short time gamma rays are emitted as the atom returns to a stable state. Gamma rays emitted due to this neutron capture reaction are labeled capture gamma rays. In wireline well logging operations, as the neutron source is pulsed and the measurements made, the subsurface well logging instrument is continuously pulled up through the borehole. This makes it possible to evaluate formation characteristics over a range of depths.
Depending on the material composition of the earth formations proximal to the instrument, the thermal neutrons can be absorbed, or “captured”, at various rates by certain types of atomic nuclei in the earth formations. When one of these atomic nuclei captures a thermal neutron, it emits a gamma ray, which is referred to as a “capture gamma ray”.
Prior art methods exist for determining attributes of a formation from logging results. See, for example, U.S. Pat. No. 4,712,424, to Herron, U.S. Pat. No. 4,394,574, to Grau et al., U.S. Pat. No. 4,390,783, to Grau, SPE 7430, SPE9461, and U.S. Pat. No. 5,471,057, to Herron.